The present invention relates generally to exposure apparatuses and methods, and more particularly to an exposure apparatus and method for projecting and exposing an object, such as a single crystal substrate for a semiconductor wafer, a glass plate for a liquid crystal display (“LCD”).
The fabrication of a device, such as a semiconductor device, an LCD device, and a thin film magnetic head using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a circuit pattern formed on a mask or reticle (these terms are used interchangeably in this application) onto a wafer and the like, thereby transferring the circuit pattern.
The projection exposure apparatus has been required to project and expose a circuit pattern on a reticle onto a wafer with higher resolution along with the finer process and higher density of integrated circuits devices. The critical dimension or resolution transferable in the projection exposure apparatus is in proportion to a wavelength of light used for the exposure and in reverse proportion to a numerical aperture (NA) in the projection optical system. Therefore, recent light sources have been in transition from the ultra-high pressure mercury lamp (including g-line (with a wavelength of about 436 nm) and i-line (with a wavelength of about 365 nm)) to a KrF excimer laser with a shorter wavelength (i.e., a wavelength of about 248 nm) to the ArF excimer laser (with a wavelength of about 193 nm), and practical use of the F2 laser (with a wavelength of about 157 nm) is also being promoted. A further expansion of the exposure area has been also required.
In order to satisfy these requirements, a step-and-repeat exposure apparatus (also referred to as a “stepper”) for entirely projecting and exposing an approximately square exposure area onto a wafer with a reduced exposure area has been replaced mainly with a step-and-scan exposure apparatus (also referred to as a “scanner”) for accurately exposing a wide screen of exposure area through a rectangular slit with relatively and quickly scanning the reticle and the wafer.
In exposure, the scanner uses a surface-position detector in an oblique light projection system to measure a surface position at a certain position on the wafer before the exposure slit area moves to the certain position on the wafer, and correctingly accords the wafer surface with an optimal exposure image-surface position when exposing the certain position, thereby reducing influence of the flatness of the wafer. In particular, there are plural measurement points in longitudinal direction of the exposure slit, i.e., a direction orthogonal to the scan direction, at front and back stages to the exposure slit area to measure an inclination or tilt of the surface as well as a height or focus of the wafer surface position. In general, the scan exposure proceeds in both directions from the upper stage and from the back stage. Therefore, these measurement points are arranged at front and back stages to the exposure slit area so as to measure the focus and tilt on the wafer before exposure. Japanese Laid-Open Patent Application No. 9-45609 (corresponding to U.S. Pat. No. 5,750,294) discloses, for example, a method for measuring such focus and tilt.
Japanese Laid-Open Patent Application No. 6-260391 (corresponding to U.S. Pat. No. 5,448,332) proposes, as a method for measuring a surface position on a wafer in a scanner and for correction the same, an arrangement of plural measurement points on a pre-scan area other than the exposure area to measure the focus and tilt in scan and non-scan directions. Japanese Laid-Open Patent Application No. 6-283403 (corresponding to U.S. Pat. No. 5,448,332) proposes as a method for measuring the focus and tilt in the scan and non-scan directions and for driving and correcting the same, by arranging plural measurement points in the exposure area.
A description will be given of these proposals with reference to FIGS. 10 and 11. Here, FIG. 10 is a schematic sectional view of focus and tilt measurement points FP1 to FP3 on the wafer 1000. FIG. 11 is a schematic sectional view showing the wafer 1000 that has been driven to an optimal exposure image-surface position based on the measurement results. Referring to FIG. 10, the focus and tilt are sequentially measured at the measurement points FP1 to FP3 on the wafer 1000. A pre-scan plane PMP is calculated based on the measurement results from the measurement points FP1 to FP3, and the orientation of the wafer is driven and adjusted to the best focus plane BFP in moving the wafer 1000 to the exposure position or exposure slit 2000, as shown in FIG. 11.
However, the recent increasingly shortened wavelength of the exposure light and the higher NA of the projection optical system have required an extremely small depth of focus (“DOF”) and a stricter accuracy with which the wafer surface to be exposed is aligned to the best focus position BFP or so-called focus accuracy.
In particular, they have also required stricter measurement and precise correction of the tilt of the wafer surface in the scan direction or width direction of the exposure slit. A wafer having an insufficiently flat surface has disadvantageous focus detection accuracy in the exposure area. For example, when the exposure apparatus has a DOF with 0.4 μm, the flatness of the wafer requires several nanometer order, for example, the flatness of the wafer needs 0.08 μm where it is one-fifth as long as the DOF, or 0.04 μm where it is one-tenth as long as the DOF.
In addition, while a surface-position detector in the oblique light projection system measures the wafer's surface position before the area hangs over the exposure slit, the measurement timing is discrete and no information is available or considered about the wafer's flatness between two timings. As a result, there is no information available between timings of the flatness of the wafer.
For example, this measurement timing is at an interval of 3 mm on the wafer 1000 in the scan direction as shown in FIG. 12. Then, the wafer 1000 has such an insufficient flatness due to lack of the information for a distance of 3 mm, e.g., between points P1 to P2 in FIG. 12 that the front position may offset by Δ from the pre-scan plane PMP calculated by the measurement at the interval of 3 mm. Here, FIG. 12 is a schematic sectional view showing an offset of flatness between the pre-scan plane PMP and the wafer 1000.
In exposure, the pre-scan plane PMP is adjusted to the best focus plane BFP, and the exposure in FIG. 12 needs a shift by the amount of Δ. This shift occurs in a direction orthogonal to the scan direction as well as the scan direction. This results from an arrangement of measurement points in the above oblique light projection system, rather than the measurement timing.
The finer measurement timing in the scan direction and the increased number of measurement points in the oblique light projection system would reduce an offset error, but might disadvantageously lower the throughput due to the deteriorated scan speed in exposure time, increase measurement time, rise cost together with the complicated apparatus structure, and grows likelihood of troubles.